During the normal delivery and birthing process, most obstetricians monitor the intrauterine pressure of the obstetrics patient. These various pressure measuring devices detect changes in the intrauterine pressure, usually caused by the onset of contractions during birth.
In the past, intrauterine pressure was measured by catheters, balloons and pressure transducers introduced into the uterus. These devices gave generally accurate relative pressure readings; however, they were invasive and caused much discomfort to the obstetrics patient.
Recently, a noninvasive method of measuring intrauterine pressure has been developed. The apparatus for practicing this noninvasive method is termed a "tokodynamometer". By pressing a flat plate on the outside surface of the abdomen, the wall of the abdomen turns into a flat diaphragm. A pressure sensitive area is located in the center region of the plate and moves in response to the corresponding intrauterine pressure exerting force upon the pressure sensitive area on the plate. A strain gauge transducer produces an output signal responsive to the movement of the pressure sensitive area, and that signal thus generally indicates the intrauterine pressure producing the force-related signal.
The foregoing kind of tokodynamometer is the subject of U.S. Pat. No. 4,640,295 to Isaacson and an article entitled "The GuardRing Tokodynamometer", by C. N. Smythe, Journal of Obstetrics and Gynecology, Volume 64, pages 59-66, (1957). However, these tokodynamometers have not been found useful to accurately measure intrauterine pressure. Tokodynamometers of the kind shown in the foregoing patent rely on a force summing element which requires mechanical movement to operate a strain gauge or other displacement-driven transducer. The mechanical displacement required by the force-summing element and by the transducer itself reduced the linearity of the measuring devices, and adversely affected calibration to such an extent that they are not useful for measuring absolute force. Consequently, the prior apparatus is only accurate with respect to periodicity of contractions, but not for the absolute amplitude thereof.
There is, accordingly, a need for a tokodynamometer which accurately measures intrauterine pressure and movement in addition to periodicity of contractions. Moreover, if the accurate intrauterine pressure is measured, the force responsible for this pressure can then be integrated to calculate the total work of a contraction and display this calculation for the obstetrician to see as the contraction occurs.